The microcirculation of mammals will be studied in terms of direct simultaneous A-V pressure measurements and data on flow and dimensions obtained in-situ and in-vivo. This data will be applied to relate microvascular hemodynamic phenomena to systemic cardiovascular effects. Development of methods for the study of pharmacological action will be continued. Specifically, the following techniques will be further improved and applied to the study of pharmacological effects: a) Development of a television velocity measuring system which provides simultaneous data on flow in one arteriole and one capillary. b) Development of an on-line quantitative method for studying the diffusion of dyes from the microcirculation. The above methods will be utilized in conjunction with pressure measuring techniques to characterize the following phenomena in the microvessel of the cat and rabbit omentum: a) Effect of sympathetic stimulation. b) Effect of topical applications of histamine. The study of the relationship between micro- and macroscopic hemodynamic phenomena will be continued.